Gasification of carbonaceous materials in an oxygen-starved condition produces syngas (also known as synthesis gas; also known as producer gas) comprising tar. Presence of tar in syngas poses major technical obstacle in gasification process causing fouling, plugging of downstream processes and equipment. Condensing tar can dramatically foul gas cleaning equipment and liquid tar droplets that enter prime movers hamper the operation of these end-use applications of the syngas. Tar in syngas may also greatly impact wastewater management. If tar and condensed water are mixed, e.g., in conventional water-based gas cleaning systems, it may create an often costly and difficult water treatment problem. In order to have a syngas acceptable for downstream processes and equipment content of tar in syngas has to be reduced. Several methods of reduction or removal of tar have been disclosed in the published art that include both physical and chemical treatment. Physical treatments for tar removal include use of filter and electrostatic tar removal. Chemical treatments include both catalytic and non-catalytic methods. One method of reducing tar content of syngas is thermal destruction in which tar undergoes one or both of partial oxidation and thermal cracking. See for example: “Tar reduction through partial combustion of fuel gas,” Houben, M. P, Lange, H. C. de & Steenhoven, A. A. van, Fuel, vol. 84, pp 817-824, 2005; “Analysis of hydrogen-influence on tar removal by partial oxidation,” Hoeven, T. A. van der, Lange, H. C. de & Steenhoven, A. A. van, Fuel, vol. 85, pp 1101-1110, 2005.
In this method, tar containing syngas produced from a gasifier unit is passed through a treatment zone or unit wherein an oxygen-containing gas is added. A high temperature in is attained in this unit in order to accomplish tar cracking and/or partial oxidation. Thus James T. Cobb, Jr. (“Production of Synthesis Gas by Biomass Gasification,” James T. Cobb, Jr, Proceedings of the 2007 Spring National AIChE Meeting, Houston, Tex., Apr. 22-26, 2007) describes a Consutech Gasifier (BRI Energy LLC), first stage of which is a standard step-grate combustor (frequently used as an MSW incinerator) that operates as a gasifier at 950° F. using oxygen-enriched air. The second stage is a heat treater that operates at 2000-2250° F. and uses minimal oxygen to crack tars.
WO 2009/154788 describes a two stage gasifier in which gaseous product from the first stage moves to the second stage. Pure oxygen is introduced into the second stage to raise the temperature to about 1750 to about 2250° F. in order to accomplish one or more of partial oxidation and cracking of tar contained in the gaseous stream from the first stage.
The above described thermal treatment method has been shown to be effective in reducing tar content of syngas in small size unit. There remains a need for developing knowledge for scale-up of this thermal treatment process in order to accomplish effective tar removal in large scale units.